Loop Edit bug fix

While using Add Notes in Loop Edit mode, notes are played before placement by holding them in place.  This allows different rhythms or chord inversions to be tested quickly.

If a note overlaps the end of the loop region while being played, the NoteOff event is not reached.  This can cause the note to hang and continue playing.

Fixed by adding a method to the PlaybackBrush class that retrieves the current playingNotes List and clears it.  This method is now called in the End of Loop processing and NoteOff events are sent to the synth for all notes in the List.

Fix will be available in version 6 and later.